Radio is a technology that allows for the transmission of signals by modulation of electromagnetic waves. These waves travel (propagate) through the air and the vacuum of space equally well, not requiring a medium of transport.

When a radio wave passes a wire, it induces a moving electric charge (voltage) that can be transformed into audio or other signals that carry information.
Although the word 'radio' is used to describe this phenomenon, the transmissions which we know as television, radio, radar, and cell phone are all in the class of radio frequency emissions.

It was Heinrich Rudolf Hertz who, between 1886 and 1888, first validated Maxwell's theory through experiment, demonstrating that radio radiation had all the properties of waves (now called Hertzian waves), and discovering that the electromagnetic equations could be reformulated into a partial differential equation called the wave equation.

In 1893 in St. Louis, Missouri, Nikola Tesla made the first public demonstration of radio communication. Addressing the Franklin Institute in Philadelphia and the National Electric Light Association, he described and demonstrated in detail the principles of radio communication. The apparatus that he used contained all the elements that were incorporated into radio systems before the development of the vacuum tube.

In 1894 British physicist Sir Oliver Lodge demonstrated the possibility of signalling using radio waves using a detecting device called a coherer, a tube filled with iron filings which had been invented by Temistocle Calzecchi-Onesti at Fermo in Italy in 1884. Edouard Branly of France and Alexander Popov of Russia later produced improved versions of the coherer. Popov, who developed a practical communication system based on the coherer, is often considered by his own countrymen to have been the inventor of radio.

In 1896Guglielmo Marconi was awarded what is sometimes recognised as the world's first patent for radio with BritishPatent 12039, Improvements in transmitting electrical impulses and signals and in apparatus there-for. In 1897 in the USA, some key developments in radio's early history were created and patented by Nikola Tesla. The US Patent Office reversed its decision in 1904, awarding Guglielmo Marconi a patent for the invention of radio, possibly influenced by Marconi's financial backers in the States, who included Thomas Edison and Andrew Carnegie. In 1909 Marconi, with Karl Ferdinand Braun, was also awarded the Nobel Prize in Physics for "contributions to the development of wireless telegraphy". However, Tesla's patent (number 645576) was reinstated in 1943 by the US Supreme Court, shortly after his death. This decision was based on the fact that there was prior work existing before the establishment of Marconi's patent. Some believe it was apparently made for financal reasons, to allow the US Government to avoid having to the pay damages that were being claimed by the Marconi Company for use of its patents during World War I (ignoring the prior establish work).

Marconi opened the world's first "wireless" factory in Hall Street, Chelmsford, England in 1898, employing around 50 people. Around 1900, Tesla opens the Wardenclyffe Tower facility and advertises services. By 1903, the ariel structure neared completion. Various theories exist on how Tesla intended to achieve the goals of this wireless system (reportedly, a 200 kW system). Wardenclyffe in operation may have allowed secure multichannel transceiving of information and may have allowed universal navigation, time synchronization, and a global location system.

On Christmas Eve, 1906, Reginald Fessenden (using his heterodyne principle) transmitted the first radio audio broadcast in history from Brant Rock Station, Massachusetts. Ships at sea heard a broadcast that included Fessenden playing the song O Holy Night on the violin and reading a passage from the Bible. The world's first regular wireless broadcasts for entertainment commenced in 1922 from the Marconi Research Centre at Writtle near Chelmsford, England, which was also the location of the world's first "wireless" factory.

Early radios ran the entire power of the transmitter through a carbon microphone. In the 1920s, amplifying vacuum tubes revolutionized both radio receivers and radio transmitters.

In 1960, Sony introduced the first transistorized radio, small enough to fit in a vest pocket, and able to be powered by a small battery. It was reliable, because there were no tubes to burn out. Over the next twenty years, transistors displaced tubes almost completely except for very high power, or very high frequency.

In the late 1960s, the U.S. long-distance telephone network began to convert to a digital network, employing digital radios for many of its links.

In the 1970s, LORAN became the premier radio navigation system. Soon, the U.S. Navy experimented with satellite navigation, culminating in the invention and launch of the GPS constellation in 1987.

In the early 1990s, amateur radio experimenters began to use personal computers with audio cards to process radio signals. In 1994, the U.S. Army and DARPA launched an aggressive, successful project to construct a software radio that could become a different radio on the fly by changing software.

Before the advent of television, commercial radio broadcasts included not only news and music, but dramas, comedies, variety shows, and many other forms of entertainment. Radio was unique among dramatic presentation that it used only sound. For more, see radio programming.

There are a number of uses of radio:

Audio

The oldest form of audio broadcast was marine radio telegraphy. A continuous wave, or CW, was switched on and off by a key to create Morse code, which was heard at the receiver as an intermittent tone. CW is still used, these days primarily by amateur radio operators (hams).

AM radio sends music and voice. AM radio uses amplitude modulation, in which higher air-pressure at the microphone causes higher transmitter power. Transmissions are affected by static because lightning and other sources of radio add their radio waves to the ones from the transmitter.

FM radio sends music and voice, with higher fidelity than AM radio. In frequency modulation, a higher air-pressure at the microphone turns into a higher transmitted frequency. FM is transmitted as Very High Frequency radio waves (VHF -- 30MHz to 300MHz). There are more frequencies available at higher frequencies, so there can be more stations, each sending more information. Another effect is that the shorter radio waves act more like light, travelling in straight lines that are not reflected back towards the Earth by the ionosphere, resulting in a shorter effective reception range. FM receivers are subject to the capture effect, which causes the radio to only receive the strongest signal when multiple signals appear on the same frequency.

FM Subcarrier services are secondary signals transmitted "piggyback" along with the main program. Special receivers are required to utilize these services. Analog channels may contain alternative programming, such as reading services for the blind, or background music. In some extremely crowded metropolitan areas, the subchannel program might be an alternate foreign language radio program for various ethnic groups. Subcarriers can also transmit digital data, such as station identification, the current song's name, web addresses, or stock quotes. In some countries, FM radios automatically retune themselves to the same channel in a different district by using sub-bands.

Marine and aviation voice radios use VHF AM. AM is used so that multiple stations on the same channel can be received. (Use of FM would result in stronger stations blocking out reception of weaker stations due to FM's capture effect). Aircraft are often so high that their radios can see hundreds of miles, even though they are using VHF.

Government, police, fire and commercial voice services use narrowband FM on special frequencies. Fidelity is sacrificed to use a smaller range of radio frequencies, usually five kilohertz of deviation (5 thousand cycles per second) for maximum pressure, rather than the 16 used by FM broadcasts and TV sound.

Civil and military HF (high frequency) voice services use shortwave radio to contact ships at sea, aircraft and isolated settlements. Most use single sideband voice (SSB), which uses less bandwidth than AM. SSB sounds like ducks quacking on an AM radio. Viewed as a graph of frequency versus power, an AM signal shows power where the frequencies of the voice add and subtract with the main radio frequency. SSB cuts the bandwidth in half by sacrificing the carrier and (usually) lower sideband. This also makes the transmitter about three times more powerful, because it doesn't need to transmit the unused carrier and sideband.

Cell phones transmit to a local cell radio, which connects to the public service telephone network through an optic fiber or microwave radio. When the phone leaves the cell radio's area, the central computer switches the phone to a new cell. Cell phones originally used FM, but now most use various digital encodings.

Television sends the picture as AM, and the sound as FM, on the same radio signal.

Digital television encodes three bits as eight strengths of AM signal. The bits are sent out-of-order to reduce the effect of bursts of radio noise. A Reed-Solomon error correction code lets the receiver detect and correct errors in the data. Although any data could be sent, the standard is to use MPEG-2 for video, and five CD-quality (44.1 kilo-sample/sec) digital channels (center, left, right, left-back and right back). With all this, it takes only half the bandwidth of an analog TV signal because the video data is compressed.

Navigation

All satellite navigation systems use satellites with precision clocks. The satellite transmits its position, and the time of the transmission. The receiver listens to four satellites, and can figure its position as being on a line that is tangent to a spherical shell around each satellite, determined by the time-of-flight of the radio signals from the satellite. A computer in the receiver does the math.

Loran systems also used time-of-flight radio signals, but from radio stations on the ground.

VOR systems (used by aircraft), have two transmitters. A directional transmitter scans like a lighthouse at a fixed rate. When the directional transmitter is facing north, an omnidirectional transmitter pulses. An aircraft can get readings from two VORs, and locate its position at the intersection of the two beams.

Radio direction-finding is the oldest form of radio navigation. Before 1960 navigators used movable loop antennas to locate commercial AM stations near cities. In some cases they used marine radiolocation beacons, which share a range of frequencies just above AM radio with amateur radio operators.

Radar detects things at a distance by bouncing radio waves off them. The delay caused by the echo measures the distance. The direction of the beam determines the direction of the reflection. The polarization and frequency of the return can sense the type of surface.

Navigational radars scan a wide area two to four times per minute. They use very short waves that reflect from earth and stone. They are common on commercial ships and long-distance commercial aircraft.

General purpose radars generally use navigational radar frequencies, but modulate and polarize the pulse so the receiver can determine the type of surface of the reflector. The best general-purpose radars distinguish the rain of heavy storms, as well as land and vehicles. Some can superimpose sonar data and map data from GPS position.

Search radars scan a wide area with pulses of short radio waves. They usually scan the area two to four times a minute. Sometimes search radars use the doppler effect to separate moving vehicles from clutter.

Targeting radars use the same principle as search radar but scan a much smaller area far more often, usually several times a second or more.

Weather radars resemble search radars, but use radio waves with circular polarization and a wavelength to reflect from water droplets. Some weather radar use the doppler to measure wind speeds.

Emergency services

emergency position-indicating rescue beacons (EPIRBs), emergency locating transmitters or personal locator beacons are small radio transmitters that satellites can use to locate a person or vehicle needing rescue. Their purpose is to help rescue people in the first day, when survival is most likely. There are several types, with widely-varying performance.

Microwave dishes on satellites, telephone exchanges and TV stations usually use quadrature amplitude modulation (QAM). QAM sends data by changing both the phase and the amplitude of the radio signal. Engineers like QAM because it packs the most bits into a radio signal. Usually the bits are sent in "frames" that repeat. A special bit pattern is used to locate the beginning of a frame.

IEEE 802.11, the radio network standard, has stations with digital tuners. They start off by contacting a central control node, which tells the nodes about each other so they can communicate privately. Nodes move through many frequencies. They use a pseudo-random number generator to select the next frequency.

Radio teletypess usually operate on short-wave (HF) and are much loved by the military because they create written information without a skilled operator. They send a bit as one of two tones. Groups of five or seven bits become a character printed by a teletype. These are classically used by the military and weather services.

Aircraft use a 1200 Baud radioteletype service over VHF to send their ID, altitude and position, and get gate and connecting-flight data.

Heating

Microwave ovens use intense radio waves to heat food. (Note: It is a common misconception that the radio waves are tuned to the resonant frequency of water molecules. The microwave frequencies used are actually about a factor of 10 below the resonant frequency.)

Mechanical Force

Tractor beams: Radio waves exert small electrostatic and magnetic forces. These are enough to perform station-keeping in microgravity environments.

Space drive: Radiation pressure from intense radio waves has been proposed as a propulsion method for interstellar probes. Since the waves are long, the probe could be a very light-weight metal mesh, and thus achieve higher accelerations than if it were a light sail.

Other

Amateur radio is an emergency and public-service radio service provided by enthusiasts who purchase or build their own equipment. It operates in a large number of narrow bands throughout the radio spectrum. Radio amateurs use all forms of encoding, including obsolete and experimental ones. Several forms of radio were pioneered by radio amateurs and later became commercially important, including FM, single-sideband AM, digital packet radio and satellite repeaters.